The kaolin group minerals comprise kaolinite, nacrite, dickite and halloysite, and are among the most common clay minerals in nature. They have a 1:1 layered structure, that is, each layer consists of one tetrahedral silicate sheet and one octahedral sheet, with two-thirds of the octahedral sites occupied by aluminium. Kaolinite, nacrite and dickite all have the ideal chemical composition: EQU Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4
They differ from one another only in the manner in which the 1:1 layers are stacked. Halloysite, in its fully hydrated form, has the ideal chemical composition: EQU Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4 O2H.sub.2 O
and differs from the other three members of the group by including molecular water in the interlayer.
Of the kaolin group minerals, kaolinite is the most abundant and has received most attention in terms of its structure, properties and industrial applications. However, because of its close similarity with the aforementioned polytypes, many of the properties and uses described for kaolinite apply equally to the other polytypes. Consequently, for the purposes of expediency, the following disclosure will be restricted primarily to kaolinite and halloysite but it should be borne in mind, as it will be readily appreciated by those skilled in the art, that the invention applies equally to nacrite and dickite.
Naturally occurring kaolins typically have a wide range of particle sizes, particle crystallinity, minor element composition(s) and chemical reactivity for intercalation reactions. Kaolins sorted into a size range of 0.5-2.0 mm typically have a specific surface of about 5 m.sup.2 g.sup.-1 and a cation exchange capacity of 10 meq./100 gm or less. These, and other properties, such as opacity and rheology, make kaolins suitable for a wide range of uses including paper coatings and fillers, pottery, porcelain and sanitaryware production and fillers in paints and rubbers. These properties however do not allow kaolins to be readily utilised in other uses as described hereinafter. However, if their specific surface and/or cation exchange capacities could be increased, their usefulness would be increased and thus they could then be used in many other applications including use as catalysts, metal scavengers, carriers and absorbents. In view of this, there has been considerable and ongoing interest in finding a process for delaminating or increasing the surface area of the layered kaolin structure as this would have the potential of making available for reaction large surface areas between the layers. To date, delamination has not been demonstrated despite substantial research on the intercalation of kaolinite and its polytypes. Recent research by N. Lahav (1990)!, Clays and Clay Minerals 38, 219-222)! suggested a stable suspension of delaminated kaolinite which had been treated with dimethylsulfoxide and ammonium fluoride in aqueous solution. The result was inferred on the basis of change of particle size and there was no evidence of the independent existence of the kaolinite reaction product.
It is therefore an object of the present invention to provide derivatives of the kaolin group minerals which have higher specific surfaces and/or higher cation exchange capacities than the kaolin group minerals per se.